Marble supports opening .osm files and editing them (as well as .kml files) in the "Edit Maps" Panel. In this task retrieving data for editing should be simplified by implementing a download option for the current viewport (visible map portion) similar to the JOSM editor.

- Add a button to the "Edit Maps" Panel to trigger the automatic download
- Have it perform safety checks first: The currently visible map portion must be small, there should not be any unsaved data
- Query data from OSM, see https://wiki.openstreetmap.org/wiki/API_v0.6#Retrieving_map_data_by_bounding_box:_GET_.2Fapi.2F0.6.2Fmap on how to create an appropriate region
- Use either QNetworkManager or HttpDownloadManager (whichever is easier) to download the data, ideally providing a progress dialog during the download
- Either just keep the data in memory and open from there, or (probably easier) save to a temporary file and open that